Emission of acid gases is detrimental to the environment. CO2 causes the so-called greenhouse effect. H2S is damaging to health, causes stench nuisance and can form acid rain. In the state of the art, many methods for selectively removing acid gases from gas mixtures have already been described. A frequently used method is a gas treatment process in which the acid gases are absorbed in a liquid. Also, it has long since been known (for instance from U.S. Pat. No. 1,990,217, U.S. Pat. No. 2,176,441 and U.S. Pat. No. 3,042,483) that weakly acid gases, such as CO2, can be removed from gas mixtures by washing with a solution of imino acids, amino acids or tertiary N-acids or salts thereof. These publications show that, preferably, solutions with a high concentration of these acids or salts should be taken up. However, at high concentrations it is experienced as a drawback that precipitates of the acids, salts and/or reaction products are formed.
An adapted method for absorption of CO2 is described in U.S. Pat. No. 5,749,941, in which the gas mixture and the absorption liquid (i.e. a solution of amino salts) are separated by a hydrophobic membrane over which the gas exchange takes place. Here, too, a high concentration of amino salts is preferable, but physically impossible, because precipitation of the amino acids causes clogging of the membrane.